In automotive finishing in particular, though also in other sectors where there is a desire for coatings featuring a good decorative effect and at the same time affording good protection against corrosion, it is known to provide substrates with two or more coating films arranged atop one another.
Multicoat paint systems are applied preferably in accordance with what is called the basecoat/clearcoat process; that is, a pigmented basecoat material is applied first and, following a short flash-off time without a baking step (wet-on-wet process), is coated over with a clearcoat material. Subsequently, basecoat and clearcoat are baked together.
The basecoat/clearcoat process has acquired particular importance in connection with the application of automotive metallic effect finishes.
Economic and environmental considerations have resulted in attempts to use aqueous basecoating compositions in the production of multicoat finishes.
The coating compositions for producing these basecoat films must be capable of being processed by the nowadays customary, rational wet-on-wet process: that is, following a very short initial drying period without a baking step, they must be capable of being coated over with a transparent topcoat, without exhibiting defects in their visual appearance, such as, for example, those known as pinholes.
Furthermore, the coating material must also exhibit sufficient stability on storage. A typical test is the storage of the material at 40° C.
With metallic effect coatings of the basecoat/clearcoat type, furthermore, there are also other problems to be solved. The metallic effect is critically dependent on the orientation of the metallic pigment particles in the coating film. A metallic effect basecoat material that can be processed in the wet-on-wet process, accordingly, must give coating films in which the metallic pigments, following application, are present in a favorable spatial orientation, and in which this orientation is fixed so quickly that it cannot be disrupted in the course of the further coating operation. Parameters suitable for characterizing the metallic effect are the lightness of the hue, and the flop index.
The laid-open specification DE 40 28 386 A1 discloses water-thinnable coating compositions which comprise a water-dilutable polyurethane resin binder, pigment particles, a thickener, and a polyamide rheological assistant. Coating materials of this kind exhibit an inadequate metallic effect and an increased tendency toward sedimentation after storage at elevated temperature (40° C.).
US 2004/0039097 A1 discloses a water-based metallic coating material that likewise comprises polyamide rheological assistants.
EP 1 153 989 A1 discloses a water-based, metallic coating composition which as well as a resin composition and a pigment is also composed of metal silicate and a polyamide resin rheological assistant. Depending on the binder combination, however, the presence of phyllosilicates can lead to pinholes.
It is an object of the present invention to provide a new coating composition that does not have the disadvantages of the prior art. This composition is to have improved application properties. Thus pinhole formation is to be reduced without a negative influence on the metallic effect. Furthermore, the sedimentation stability of the composition is to be ensured.
The terms “pinholes” and “metallic effect” are known to the skilled worker and defined, for example, in Römpp Lexikon, Lacke and Druckfarben, Georg Thieme Verlag 1998.
Surprisingly it has been found that this object can be achieved through the combination of polyamides and poly(meth)acrylic acids as rheological assistants.